1. Field of the Invention
The present invention relates to a method and system for integrated link adaptation and power control in wireless networks to improve error and throughput performance of a wireless network.
2. Description of the Related Art
As telecommuting and Internet access become increasingly popular, customer demand for broadband network services is increasing. In the very near future, broadband services are also expected to support real-time, multimedia services such as voice, image and video. Wireless access is one of the approaches to providing such services. In particular, the European Telecommunications Standards Institute is in the process of establishing the protocol standards for the Enhanced Data rates for GSM Evolution (EDGE) system as a third generation of wireless networks for high-speed services. Using packet-switching technology, and multiple modulation and coding levels (to be referred to as modulation levels below for brevity), the EDGE system employs a link-adaptation technique to adapt packet transmissions to one of several modulation levels where the highest data rate can exceed 384 Kbits/sec.
The idea of link adaptation is to adapt the modulation encoding levels according to the channel and interference conditions in order to improve data throughput. For example, when the channel and interference conditions are poor, a low modulation level (i.e., few information bits per symbol) and/or heavy coding should be used in a packet transmission to enable correct signal detection. On the other hand, if the channel situations are more favorable, a high modulation level and/or light coding can be used to increase the data rate.
Due to unreliable radio links, it is challenging to assure a quality of service (QoS) in terms of packet error rate (PER) in a wireless network. For real-time services, such as IP voice, music and video, stringent delay requirements severely limit or even preclude re-transmission of lost packets. Therefore, tight delay requirements often translate into stringent requirements for the PER. As a result, in order to support such real-time services, it is important to design wireless networks such that the required QoS can be delivered to the users.
Currently, it is known that link adaptation is helpful in delivering a particular QoS. Specifically, when a channel condition is poor, transmitters can lower modulation levels to decrease the requirement of the signal-to-interference-plus-noise ratio (SINR) for correct signal detection. Lowering the SINR requirements increases the probability of successful reception, and therefore helps to meet particular PER objectives.
However, especially for interference-limited systems with sufficient traffic load, adapting even to the lowest modulation level may not always guarantee meeting the specified PER. In this case, increasing a transmission power can improve signal strength, and therefore the SINR at the receivers. Hence, power control can be viewed as performing an active role in delivering the expected PER to users, while link adaptation or adaptive modulation plays a passive (or reactive) role.
Accordingly, a key design problem for a wireless packet network, such as the EDGE system, is how to maximize the overall network throughput over the choice of modulation levels, and transmission power, subject to meeting given PER requirements.